netdev - RE: [PATCH v2] usbnet: remove generic hard_header

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Date:	Thu, 13 Feb 2014 16:56:58 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Emil Goode' <emilgoode@...il.com>,
	Steve Glendinning <steve.glendinning@...well.net>,
	Oliver Neukum <oneukum@...e.de>,
	Bjørn Mork <bjorn@...k.no>,
	"David S. Miller" <davem@...emloft.net>,
	Freddy Xin <freddy@...x.com.tw>,
	Eric Dumazet <edumazet@...gle.com>,
	Ming Lei <ming.lei@...onical.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Liu Junliang <liujunliang_ljl@....com>,
	Octavian Purdila <octavian.purdila@...el.com>
CC:	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2] usbnet: remove generic hard_header_len check

From: Of Emil Goode
> This patch removes a generic hard_header_len check from the usbnet
> module that is causing dropped packages under certain circumstances
> for devices that send rx packets that cross urb boundaries.
> 
> One example is the AX88772B which occasionally send rx packets that
> cross urb boundaries where the remaining partial packet is sent with
> no hardware header. When the buffer with a partial packet is of less
> number of octets than the value of hard_header_len the buffer is
> discarded by the usbnet module.
...
> diff --git a/drivers/net/usb/ax88179_178a.c b/drivers/net/usb/ax88179_178a.c
> index d6f64da..955df81 100644
> --- a/drivers/net/usb/ax88179_178a.c
> +++ b/drivers/net/usb/ax88179_178a.c
> @@ -1118,6 +1118,10 @@ static int ax88179_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
>  	u16 hdr_off;
>  	u32 *pkt_hdr;
> 
> +	/* This check is no longer done by usbnet */
> +	if (skb->len < dev->net->hard_header_len)
> +		return 0;
> +

The ax88179 driver can also receive ethernet frames that cross the
end of rx URB.
It should have the code to save the last fragment (of a urb) until
the next data arrives, and then correctly merge the fragments.

It is likely that any sub-driver that sets the receive urb length
to a multiple of 1k (rather than leaving it at hard_hdr+mtu) can defrag
rx data that crosses urb boundaries.

	David



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